The power supply, such as a DC to DC converter, for a vehicle electronic system typically receives electrical power from the vehicle battery. Unfortunately, the power supply may be exposed to hazardous voltages. For example, an "alternator load dump" causes damaging voltage spikes that can cause catastrophic failure of the vehicle electronics. Each voltage spike may have a magnitude of 150 volts with a duration of several milliseconds. Typically, these voltage spikes occur every few seconds. An alternator load dump is caused by an abrupt change in the alternator load. For example, the headlights being turned "off" or a poor battery connection can abruptly change the alternator load.
One solution to protecting the power supply for the vehicle's electronic system from damaging voltage spikes is shown in the circuit of FIG. 1. During normal operation, the illustrated circuit provides battery voltage to an electrical load, R-load (representative of the power supply). During abnormal conditions, e.g. hazardous voltage spikes, a Zener diode, D, effectively "clamps" the voltage impressed across R-load at the Zener diode voltage.
However, several problems are inherent with the design of the illustrated circuit. For example, approximately a 3 volt drop occurs across the collector-emitter junction of the bi-polar Darlington transistor, Q. Consequently, the 13.8 volts from a fully charged 12 volt battery is reduced to 10.8 volts, which may not be sufficient to power the vehicle electronics. Further, the transistor dissipates a great amount of power, heating the transistor. Therefore a heat sink is required dissipate the thermal energy. The heat sink adds cost and bulk to the circuit, rendering electronic packaging difficult.
The present invention is directed to overcoming one or more of the problems as set forth above.